Process for the recovery of selenium from minerals and/or acidic solutions

ABSTRACT

A process and method for recovering elemental selenium, selenite or selenate from minerals and selenium salts solutions using a reducing sugar to reduce selenium before precipitating the elemental selenium, selenite and/or the selenate.

FIELD OF THE INVENTION

The present invention relates to a process and method for separating andrecovering precious metals from copper refinery anode slimes in general,and in particular to a process and method for recovering selenium fromminerals and acidic solutions.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects, features, and advantages of the presentinvention will be better appreciated from the following description ofthe embodiments, considered with reference to the accompanying drawing,wherein:

FIG. 1 is a drawing showing an outline of a process for separatingselenium and tellurium from the material and from each other, and aprocess for precipitating and recovering selenium.

FIG. 2 is a drawing showing an outline of a process for recoveringselenium from a material containing soluble Se salts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Copper refinery anode slimes, which are settled and accumulated at thebottom of the electrolytic copper refining cells, are made up ofcomponents of the anodes which are not soluble in the electrolyte. Thesecomponents contain various amounts of copper, silver, gold, sulphur(sulfur), lead, arsenic, selenium, tellurium, nickel, silica, and othercomponents. Copper present in the slimes occurs usually in the form ofmetallic copper sulphide, copper telluride or copper selenide. In thetreatment of anode slimes, impurities, for example, but not limited to,copper and nickel are removed first and selenium and telluriumthereafter, using different recovery processes.

Whenever selenium and tellurium are present simultaneously insignificant concentrations in the slime, it is important to provide aclean separation between these two elements before they are removed andrecovered. It is also important to improve the recovery of theseprecious metals, to reduce the processing expense, and to use a processthat is environmentally friendly. In the embodiment of FIG. 1, theprocess to recover selenium from a mineral includes, in general, thesteps of: (i) leaching, where the precious metals are introduced to anaqueous environment, (ii) extraction, where the metals are separated andpurified, and (iii) reduction, where the metallic ions are reduced andprecipitated from solutions. Reducing the metallic ions using reducingsugars or hydrolysis products of sugars, such as, sucrose, significantlyreduces the overall processing expense.

Referring to FIG. 1 it is shown a hydrometallurgical process 100 forseparating tellurium and selenium from a raw material feed 105containing copper, tellurium, selenium, as well as other metals andimpurities. The raw material feed 105 is first finely ground 110 andthen mixed with an aqueous solution 115, such as water, to obtain aslurry 120 containing about 20 to 40% solids. As part of an oxidation125, an alkali medium is added to the slurry and its concentrationadjusted to about 50-250 g/L. The amount of the alkali medium requiredvaries with the composition of the treated material and the particularapplication. The alkali medium can be any alkali metal hydroxide, suchas sodium, potassium or lithium hydroxide.

In FIG. 1, sodium hydroxide (NaOH) is used as the alkali hydroxide. Thealkali hydroxide added to the slurry during the oxidation 125 dissolvescertain materials from the solids present in the slurry (e.g.,leaching). For example, selenium is soluble and will migrate into thealkali solution whereas tellurium remains in the solids, forming part ofthe undissolved leaching residue. The slurry is next heated in apressure vessel while oxygen (O₂) is injected into the vessel tofacilitate oxidation of tellurium to tellurate and oxidation of seleniumto a selenium salt in an alkaline solution. The oxygen can be provided,for example, by commercial oxygen, however, air may also be used as theoxidation agent. Typically, the temperature and pressure in the pressurevessel are maintained at around 140 degrees Celsius and 140 psi, untiloxidation 125 is complete. In the presence of copper, the alkali metaland oxygen, tellurium oxidizes to tellurate (TeO₄ ²⁻, or TeO₆ ⁶⁻) andselenium to a selenium salt.

The oxidized tellurium is insoluble and remains in the solids as part ofthe undissolved leaching residue, whereas the selenium, which migratedinto the alkali hydroxide solution, remains in the alkali solution.Separation 130 of selenium salts 135 from the oxidized telluriumrequires a solid/liquid separating process, for example, filtration 130.During filtration 130 the alkali hydroxide solution is separated fromthe undissolved leaching residue, which results in a clean separationbetween selenium and tellurium. Polytetrafluoroethylene filter cloths orany other filter material can be used to separate 130 the undissolvedcomponents from the dissolved components. Once separated, the elementalselenium and elemental tellurium are each recovered separately. Therecovery of the elemental selenium, selenite and/or selenate is nextdescribed. For the recovery 131 of the elemental tellurium see copendingpatent application Ser. No. 12/494,153, filed concurrently herewith, thedetails of which are hereby incorporated herein by reference in itsentirety.

After oxidation 125 and filtration 130, the soluble selenium salts 135are made alkaline with an alkali hydroxide solution by adjusting 140 theconcentration of the alkaline solution to about 100-400 g/L. A reducingagent is added 145 to the alkaline solution to facilitate reduction ofthe selenium. Any of the available reducing sugars, sacharrides, orhydrolysis products of sugars, such as fructose, sucrose, polyhydroxyaldehydes and ketones can be used as the reducing agent, with sucrosebeing used in the embodiment of FIG. 1. The alkaline solution, which is,for example but not limited to, sodium hydroxide, is initially heated toa temperature of about 95 degrees Celsius to initiate 150 reduction ofthe selenium in the alkaline solution. The temperature of the alkalinesolution is then increased to about 140 degrees Celsius, for example, tocomplete the reduction process 155 of the selenium. Under theseconditions selenium is effectively reduced from Se⁶⁺ to substantiallySe⁴⁺. Selenates and selenites can also be reduced during this processand the reduction can be aided by a small amount of additional selenateor elemental selenium. The solution can be diluted 160 in an aqueoussolution, such as water, and allowed to cool down. The cooled alkalinesolution containing the reduced selenium is then areated 165 toprecipitate the elemental selenium. The precipitated elemental seleniumis next separated from the alkaline solution, by, for example,filtration 170. Following filtration 170, elemental selenium isrecovered 175 and the caustic solution is evaporated 180 to concentratethe alkali metal solution and recycled for use in adjusting 140 theconcentration of alkaline solution containing the selenium salt in thenext recovery operation.

In the embodiment of FIG. 2, it is shown a hydrometallurgical process200 for recovering elemental selenium from a material containing solubleselenium salts 205. The soluble selenium salts 205 are made alkalinewith an alkali hydroxide solution by adjusting 210 the concentration ofthe alkaline solution to about 100-400 g/L. A reducing agent is added215 to the alkaline solution to facilitate reduction of the selenium.Any of the available reducing sugars, sacharrides, or hydrolysisproducts of sugars, such as fructose, sucrose, polyhydroxy aldehydes andketones can be used as the reducing agent, with sucrose being used inthe embodiment of FIG. 2. The alkaline solution, which is, for examplebut not limited to, sodium hydroxide, is initially heated to atemperature of about 95 degrees Celsius to initiate 220 reduction of theselenium in the alkaline solution. The temperature of the alkalinesolution is then increased to about 140 degrees Celsius, for example, tocomplete the reduction process 225 of the selenium. Under theseconditions selenium is effectively reduced from Se⁶⁺ to substantiallySe⁴⁺. Selenates and selenites can also be reduced during this processand the reduction can be aided by a small amount of additional selenateor elemental selenium. The solution can be diluted 230 in an aqueoussolution, such as water, and then allowed to cool down. The cooledalkaline solution containing the reduced selenium is then areated 235 toprecipitate the elemental selenium. The precipitated elemental seleniumis next separated from the alkaline solution, by, for example,filtration 240. Following filtration 240, elemental selenium isrecovered 245 and the caustic solution is evaporated 250 to concentratethe alkali metal solution and recycle for use in adjusting 210 theconcentration of alkaline solution containing the selenium salt in thenext recovery operation.

Disclosed embodiments include (i.e., comprise) a process and method forrecovering elemental selenium (Se) from a soluble selenium salt in analkaline solution by first reducing selenium from Se⁶⁺ to substantiallySe⁴⁺. To facilitate reduction of the selenium, a reducing agent is addedto the solution before heating the solution to a predeterminedtemperature. The heating temperature of the solution is between 90-180degrees Celsius. After reduction, the alkali solution is cooled, whichresults in the precipitation of the elemental selenium in the alkalisolution. The reducing agent can be a reducing sugar, or a hydrolysisproduct of a sugar and the alkali solution can be an alkali hydroxidemedium.

In an embodiment the reducing agent is sucrose.

In another embodiment the alkali solution is a sodium hydroxide solutionwith a concentration of between 100-400 g/L.

In another embodiment the alkali solution is diluted with water aftercooling, and aeration is performed on the solution before precipitating.The solution can also be evaporated after selenium is recovered, inorder to recycle the alkali metal for the next recovery batch.

Disclosed embodiments also include (i.e., comprise) a method forreducing selenium in an alkaline solution by first adjusting theconcentration of the alkaline solution to be between 100-400 g/L, andthen reducing the selenium in the alkaline solution by adding a reducingagent to the solution before heating the solution to a temperature ofbetween 90-180 degrees Celsius. The reducing agent can be a reducingsugar or a hydrolysis product of a sugar.

In another embodiment, the hydrolysis product is sucrose.

Disclosed embodiments further include (i.e., comprise) a method forseparating and recovering elemental selenium from a material containingtellurium and selenium. In this method the material is leached with analkali solution causing selenium to migrate into the alkali solution.Tellurium and selenium are then oxidized by heating the leaching residueand the alkali solution to a predetermined first temperature of about140 degrees Celsius until oxidation is complete. The leaching residuewhich includes the oxidized tellurium is then separated from the alkalisolution which contains oxidized selenium (selenium salt). The seleniumsalt is made alkaline by adjusting the concentration of the alkalisolution to be about 100-400 g/L. A reducing agent is added to thealkali solution and heated to a predetermined second temperature ofabout 90-180 degrees Celsius, effectively reducing selenium from Se⁶⁺ tosubstantially Se⁴⁺. After cooling, selenium is precipitated andfiltered, and the caustic solution evaporated to concentrate and recyclethe alkali metal for the next recovery process. Tellurium can berecovered from the leaching residue from the oxidation portion through aseparate recovery process, which is the subject of copending patentapplication Ser. No. 12/494,153, filed concurrently herewith. Thereducing agent can be a reducing sugar or a hydrolysis product of asugar.

In an embodiment, the alkali solution is an alkali hydroxide solution.

In another embodiment, the alkali hydroxide solution is a sodiumhydroxide solution.

In yet another embodiment, the reducing agent is sucrose.

It should be understood that the present invention is not limited to theembodiments described therein. Rather, those skilled in the art willappreciate that various changes and modification can be made in keepingwith the principles exemplified by the illustrative embodiments.

1. A process for recovering elemental selenium (Se) from a selenium saltin an alkaline solution, the process comprising: reducing the seleniumsalt in the alkaline solution using an additional reducing agent beforeheating the alkaline solution to a predetermined temperature, whereinthe predetermined temperature is between 90 and 180 degrees Celsius;cooling the alkaline solution; precipitating elemental selenium aftercooling the alkaline solution; and recovering the elemental seleniumfrom the cooled alkaline solution.
 2. The process as claimed in claim 1,wherein the reducing agent is a reducing sugar.
 3. The process asclaimed in claim 1, wherein the reducing agent is a hydrolysis productof a sugar.
 4. The process as claimed in claim 3, wherein the hydrolysisproduct is sucrose.
 5. The process as claimed in claim 1, wherein thetemperature is about 140 degrees Celsius.
 6. The process as claimed inclaim 1, wherein the alkaline solution is an alkali hydroxide solution.7. The process as claimed in claim 6, wherein the alkali hydroxidesolution is a sodium hydroxide solution.
 8. The process as claimed inclaim 1, further comprising adjusting a concentration of the alkalinesolution using sodium hydroxide to be between 100-400 g/L before thereducing of the selenium.
 9. The process as claimed in claim 8, whereinthe alkaline solution concentration is about 200 g/L.
 10. The process asclaimed in claim 1, further comprising diluting the alkaline solutionafter the cooling.
 11. The process as claimed in claim 10, furthercomprising aerating the alkaline solution after the diluting.
 12. Theprocess as claimed in claim 11, further comprising evaporating thealkaline solution in order to concentrate the alkaline solution forrecycling.
 13. The process as claimed in claim 1, wherein the heatingthe alkaline solution to a predetermined temperature includes heatingthe alkaline solution to a first predetermined temperature and thenheating the alkaline solution to a second predetermined temperature, thesecond predetermined temperature being higher than the firstpredetermined temperature.
 14. A method for recovering selenium from asolution containing selenium salt, the method comprising: Adjusting analkali concentration of the solution about 100-400 g/L using an alkalihydroxide to generate an alkaline solution; reducing the selenium saltin the alkaline solution by: adding a reducing sugar to the alkalinesolution; and heating the alkaline solution to a predeterminedtemperature; cooling the alkaline solution; diluting the alkalinesolution after the cooling; aerating the diluted alkaline solution toprecipitate selenium; and separating the precipitated elemental seleniumfrom the diluted alkaline solution.
 15. The method as claimed in claim14, further comprising evaporating the separated alkaline solution forrecycling.